Automatically, or semi-automatically activated, injection devices have been on the market for many years. One of the first types was intended for war-times, which type was activated by merely pressing or slamming the injector against a body part thereby activating it. The main concern was to have the medicament injected as fast as possible, without much concern for the patient or for safety aspects such as unintentional firing.
Since auto-injectors began to be used by “normal” patients, such as for example asthmatics, and also since the AIDS risk has become an important issue, the handling and safety aspects have become much more important. Patent document U.S. Pat. No. 5,085,642 discloses an auto-injector comprising a preloaded compression spring acting on a plunger, which in turn acts on a syringe. Before use the needle is protected inside the injector from being contaminated. In order to activate the injector a cap has to be removed so that a firing button may be available. When the firing button is depressed, the plunger is released, whereby it pushes the needle out of the cover and the medicament is discharged with the help of the compression spring.
With the auto-injector according to U.S. Pat. No. 5,085,642, once the cap has been removed, there is a pronounced risk of unintentional misfiring if the patient depresses the firing button before the injector is placed against the intended injection location. This may be fatal if the patient for example suffers from an acute allergic attack and requires an injection immediately in order to seize the attack. Further, with this injector, the needle is completely exposed after the injection, whereby there is a pronounced risk of being pierced or scraped by the needle.
Patent document U.S. Pat. No. 5,681,291 discloses an auto-injector comprising a first and a second device for performing the injection. The first device retracts a needle cover surrounding the needle before use into the injector housing when a predetermined force is placed on the injector, thereby exposing the needle so that it can be pushed into the patient. When the needle cover has been retracted a certain distance, the second device is activated, which acts on the plunger of the syringe for injecting the medicament. The injector may also be provided with a spring for extracting the needle cover from the housing once the injector is withdrawn from the injection site, thereby covering the needle after use. The needle cover may also be locked in that extracted position.
One drawback with the injector according to U.S. Pat. No. 5,681,291 is that the injector has to be pressed actively by the patient with a rather high force in order to overcome the holding force of the needle cover and for entering the needle into the body of the patient. This force is even more pronounced when an extracting spring is used. This in connection with necessary step of pulling free a flexible arm on the outside of the injector in order to activate it makes the handling of the device very cumbersome and not user-friendly nor easy to use. The patient needs to be very active and to perform a certain number of actions, some with considerable force, in order to inject the medicament. In a crisis situation, such as described above, there may be a risk that the patient fails to handle the injector properly. The injector is also not suitable for children because of this.
Another aspect with the injector according to U.S. Pat. No. 5,681,291 is that the needle cannot be kept from becoming contaminated since the needle cover has an opening in the injection direction. The needle may of course have a removable cover or sheath, but removal of that would constitute yet a step in order to activate the injector.